2»2 



BIOLOGY OF THE LABORATORY MOUSE 



Genetic theory of transplantation. — It would be cumbersome to continue 

 to develop this theory further by diagrams. We may, however, give a table 

 which shows the percentages of susceptible mice to be expected when larger 

 numbers of genes are needed (Table 3). In this table the data already 

 shown in diagrams will be included. 



PARENT I. 



PARENT 2. 



SUSCEPTIBILITY NON-SUSCEPTIBILITY 



A a B BOTH PRESENT SUSCEPTIBILITY 



SUSCEPTIBLE 



V 



7 

 NON-SUSCEPTIBLE 



PERCENT SUSCEPTIBLE 5625 



Fig. 128. — Diagram showing the inheritance of susceptibility to transplanted tumors 

 where susceptibility is due to the simultaneous presence of two dominant genes. 



It will be noted that as the number of genes needed increases the Fi and 

 backcross with the susceptible parent give constant figures. The behavior 

 of the F2 generation and of the backcross with the non-susceptible parent is 

 quite different. As the number of genes increases the percentage of suscepti- 

 ble animals in these generations decreases with great rapidity. The decrease 

 is more rapid in the backcross than in the F2 generation and after 11 or 12 

 genes are involved would result in practically negligible occurrence of 

 susceptible animals in the former generation. 



If we now compare the results obtained by Little and Tyzzer with the 

 expectation for 14-15 genes we get the situation shown in Table 4. 



